Worry itself may be well worth worrying about. Not just for the standard reason that there is an optimal range of worry: too much or too little is inimical to health. But rather because worry poses a question for science that—to use a distinction introduced by Chomsky—is a mystery rather than a problem. The question is simple: What is worry?

A short answer is that worry is a state of anxiety coupled with uncertainty about present or anticipated problems.

This answer can be fleshed out with details of the biological correlates of worry. Briefly, when we worry and experience psychological stress there is activation of the anterior cingulate cortex and orbito-frontal cortex of the brain, which interact with each other and with the amygdala. The amygdala interacts with the hippocampus and other subcortical structures, which in turn control the hypothalamus. The hypothalamus controls the autonomic nervous system, which governs the release of epinephrine and acetylcholine, and the HPA axis, which governs the release of cortisol into the bloodstream. In short, when we worry there is an impressively complex array of interacting neural and endocrine activities that engages the whole person, brain and body. The correlation between worry and this complex biological activity is systematic and predictable.

Most of us know worry in a different way—not as a complex biological activity but rather as an unpleasant conscious experience. The experience of worry can vary from mild, as when we worry whether to take an umbrella; through deeply troubling, as when we worry over choosing a treatment for cancer; to near insanity, as when Lady Macbeth exclaims "Out, damned spot! Out, I say!—One; two; why then 'tis time to do't; —Hell is murky! —Fie, my lord, fie! a soldier and afeard?" Worry, as a conscious experience, prompts us to react, sometimes with unproductive nervous activity, other times with productive problem solving, and occasionally with compensating humor: "I'm not afraid of death," said Woody Allen, "I just don't want to be there when it happens."

Worry, then, is Janus-faced, having two complex aspects: biology and conscious experience. To answer the question "What is worry?" requires that we unite these two aspects. And therein lies the mystery.

It is natural to assume that the neural and endocrine correlates of worry are in fact the cause, or basis, of the conscious experience of worry. A scientific theory, under this assumption, should explain precisely how these biological correlates cause, or give rise to, the varied and nuanced conscious experiences that we call worry. The problem is that there is no such theory. The mystery is that, to date, there are no remotely plausible ideas.

Which is not to say there are no ideas. There are many. Perhaps certain oscillatory patterns of activity in the right neuronal circuits, certain quantum states in the right neuronal microtubules, certain degrees of informational complexity in the right reentrant thalamo-cortical loops, or certain functional properties of the right cortical and subcortical brain areas, are responsible for causing, or giving rise to, the conscious experience of worry.

These are interesting ideas to explore. But taken, in their present state, as scientific theories they bring to mind a well-known cartoon in which two geeks stand before a chalk board on which are scribbled nasty equations, followed by the phrase "and then a miracle occurs," followed by more nasty equations; one geek says to the other, "I think you should be more explicit here in step two." Unfortunately, this is the state of each idea. A reentrant thalamo-cortical loop achieves the right informational complexity … then a miracle occurs … then the conscious experience of worry results. Microtubules enjoy the right quantum states … then a miracle occurs … then the experience of worry results. There is no explanation of how or why the conscious experience of worry appears, much less why this experience has precisely the qualities it does. No one can make the leap from biology to conscious worry without waving a magic wand at precisely the point one had hoped for an explanation.

This worrisome predicament has prompted desperate responses. Perhaps conscious worries are an illusion; there are no such things, and we have been worrying over nothing. Or perhaps conscious worries are real and caused by biology, but evolution has not equipped us with the concepts we need to understand how; if so, then the biological provenance of worry will remain a mystery to us, at least until we chance upon a mutation that endows the needed concepts.

What should we be worried about? This Edge question itself implies an assumption that worry has causal powers—that if, for instance, we worry about the right topics then this worry might prompt productive problem solving.

But does the conscious experience of worry really have causal powers? If we answer yes, then we must explain how its causal powers are related to those of biology. Once again, we face an unsolved mystery. So, many biologists answer no. They assume, that is, that the brain somehow causes conscious worries, but that conscious worries, and conscious experiences more generally, cause nothing. This successfully exorcises the mystery of causation, only to introduce another mystery: How and why did conscious experiences evolve? Natural selection can only select among traits that have causal consequences for fitness. If conscious experiences have no causal consequences, then a fortiori they have no consequences for fitness, and thus are not subject to natural selection. Thus the experience of worry is not a product of natural selection—a worrisome conclusion.

So what should we be worried about? We should be worried about what we're really doing when we worry. Unless, of course, there are no worries or worries do nothing. If there are no worries, then there is nothing to worry about, and it makes no sense even to advise "Don't worry, be happy." If there are worries, but they do nothing, then you can worry or not as you please, it makes no difference. But the worry is that there really are worries, that worries really do something, and that what they are, how they arise, and what they can do is, for now, a mystery.